Pregnancy complications may increase autism risk

Embargoed until 01 July 2009

Complications during pregnancy may increase
the risk of having a child with autism, according to American
researchers.

The team reviewed 64 studies of prenatal risk
factors for autism. It is the first time a meta-analysis of the
relationship between pregnancy-related factors and risk of autism
has been carried out. The
analysis is published in the July issue of the British
Journal of Psychiatry.

Over 50 prenatal factors were examined. The
researchers found that the factors most strongly associated with an
increased autism risk are:

Being born to an older mother or father.

Having a mother who was born abroad.

Having a mother who experienced bleeding
during pregnancy.

Having a mother who experienced gestational
diabetes.

Having a mother who used medication during
pregnancy.

Being the first born - or later born in
families where there are three or more children.

The researchers put forward possible
explanations for these risk factors. For example, increased
maternal age may be associated with autism because of a higher risk
of chromosomal abnormalities in eggs.

Mothers who are born in another country may
not have natural resistance to infections in the country where they
give birth, which may increase the risk for autism. Moving to
another country may also put women under stress, which could
increase their chances of having a child who develops autism.

Bleeding during pregnancy, gestational
diabetes and medication use are also associated with increased
autism risk. Bleeding can cause foetal hypoxia (lack of oxygen to
the brain of an unborn child). Women who develop diabetes during
pregnancy experience hormonal and metabolic changes, which may
affect their baby’s health and development. Foetal development may
also be affected by some medications which can cross the placenta
during pregnancy.

The association between birth order and autism
risk is unclear. However, children with autism are more likely to
be the first-born in families with only two children. In larger
families with three or more children, they are more likely to be
born later. It is possible that parents decide not to have more
children after one has developed autism.

The researchers said there was “insufficient
evidence” to point to any one prenatal factor as being particularly
significant. However, writing in the British Journal of
Psychiatry, they said: “There is some evidence to suggest that
exposure to pregnancy complications in general may increase the
risk of autism.”